1. Field of the Invention
The invention relates to simultaneous transmission of speech and data on a mobile communications system, and it is aimed specifically at the simultaneous transmission of speech and data on a wireless circuit switched channel.
2. Description of the Prior Art
The latest mobile communications systems, like, for example, the European GSM system, are able to offer the users excellent facilities for data transfer, thus enabling the use of different data services by using the terminals of the mobile communications system. The closest prior art in regard to the invention is presented, for example, by the bearer services defined in the GSM system specifications. These bearer services offer the users the opportunity to switch between speech and data transfer, and the services have been defined as both transparent and non-transparent. The implementation of these services, however, is based on the system having separate speech and data modes for the whole transmission chain, and changing between the two modes requires exchanging several signaling messages, which is time-consuming. For example, to change from speech transfer to transferring a fax requires the sender to take the initiative by entering an appropriate command. After this, the change to the data transfer mode is signaled throughout the line, and the acknowledgment of the change is signaled back to the initiating end of the line. After this the data transfer can start. Also the transmission paths for the speech and data signals are different.
In addition, with the GSM system, a new packet data transfer service is being defined. The abbreviation for the service is GPRS (General Packet Radio Service). GPRS data is designed to be transferred by using its own channel, which is new in the system and uses a specific GPRS access control algorithm. It is also required that the new system should be able to carry GPRS data simultaneously with circuit switched data (speech, faxes or other circuit switched data) being carried on another channel. The current terminal devices have not been designed for transferring radio data simultaneously on two channels, that is, for transferring circuit switched data on a TCH channel and packet data on a GPRS channel, and that is why it is difficult to fulfill these requirements.
One clear disadvantage in the operation of the GSM system is the inefficient use of a circuit switched channel capacity. Studies show that, for example, the average activity for speech connection is 44%. So, 56% of the capacity of the circuit switched channel reserved for this purpose remains for other uses, which is mostly idle traffic. As the persons skilled in the art know, it is just this inefficient use of channel resources that has resulted in the development of the DTX function for the most recent mobile communications systems. DTX, that is, discontinuous transmission, which is used in the transmission direction, works in a way that the transmission is discontinued when there is no information, normally speech, to be sent. This function has been developed mainly for saving power. However, the circuit switched connection, that is, the traffic channel between the mobile station and the base station, remains reserved for this purpose, for example, because the receiving end must be active on this channel continuously, as there is no way of knowing when the transmission continues. The operation of the DTX will be further discussed in a description of one of the embodiments of the invention.
According to studies, in mobile communications systems the active use of a circuit switched connection in data transfer takes place in bursts even more so than in speech transfer. So the activity percentage in the transmission of data files is about 29%, and even lower in interactive data transfer, about 3.6%.